feat(安全): 强化安全策略与配置校验

- 增加 CORS/CSP/安全响应头与代理信任配置

- 引入 URL 白名单与私网开关，校验上游与价格源

- 改善 API Key 处理与网关错误返回

- 管理端设置隐藏敏感字段并优化前端提示

- 增加计费熔断与相关配置示例

测试: go test ./...

backend/internal/util/responseheaders/responseheaders.go

@@ -0,0 +1,92 @@
+package responseheaders
+
+import (
+	"net/http"
+	"strings"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+)
+
+// defaultAllowed 定义允许透传的响应头白名单
+// 注意：以下头部由 Go HTTP 包自动处理，不应手动设置：
+//   - content-length: 由 ResponseWriter 根据实际写入数据自动设置
+//   - transfer-encoding: 由 HTTP 库根据需要自动添加/移除
+//   - connection: 由 HTTP 库管理连接复用
+var defaultAllowed = map[string]struct{}{
+	"content-type":                   {},
+	"content-encoding":               {},
+	"content-language":               {},
+	"cache-control":                  {},
+	"etag":                           {},
+	"last-modified":                  {},
+	"expires":                        {},
+	"vary":                           {},
+	"date":                           {},
+	"x-request-id":                   {},
+	"x-ratelimit-limit-requests":     {},
+	"x-ratelimit-limit-tokens":       {},
+	"x-ratelimit-remaining-requests": {},
+	"x-ratelimit-remaining-tokens":   {},
+	"x-ratelimit-reset-requests":     {},
+	"x-ratelimit-reset-tokens":       {},
+	"retry-after":                    {},
+	"location":                       {},
+}
+
+// hopByHopHeaders 是跳过的 hop-by-hop 头部，这些头部由 HTTP 库自动处理
+var hopByHopHeaders = map[string]struct{}{
+	"content-length":    {},
+	"transfer-encoding": {},
+	"connection":        {},
+}
+
+func FilterHeaders(src http.Header, cfg config.ResponseHeaderConfig) http.Header {
+	allowed := make(map[string]struct{}, len(defaultAllowed)+len(cfg.AdditionalAllowed))
+	for key := range defaultAllowed {
+		allowed[key] = struct{}{}
+	}
+	for _, key := range cfg.AdditionalAllowed {
+		normalized := strings.ToLower(strings.TrimSpace(key))
+		if normalized == "" {
+			continue
+		}
+		allowed[normalized] = struct{}{}
+	}
+
+	forceRemove := make(map[string]struct{}, len(cfg.ForceRemove))
+	for _, key := range cfg.ForceRemove {
+		normalized := strings.ToLower(strings.TrimSpace(key))
+		if normalized == "" {
+			continue
+		}
+		forceRemove[normalized] = struct{}{}
+	}
+
+	filtered := make(http.Header, len(src))
+	for key, values := range src {
+		lower := strings.ToLower(key)
+		if _, blocked := forceRemove[lower]; blocked {
+			continue
+		}
+		if _, ok := allowed[lower]; !ok {
+			continue
+		}
+		// 跳过 hop-by-hop 头部，这些由 HTTP 库自动处理
+		if _, isHopByHop := hopByHopHeaders[lower]; isHopByHop {
+			continue
+		}
+		for _, value := range values {
+			filtered.Add(key, value)
+		}
+	}
+	return filtered
+}
+
+func WriteFilteredHeaders(dst http.Header, src http.Header, cfg config.ResponseHeaderConfig) {
+	filtered := FilterHeaders(src, cfg)
+	for key, values := range filtered {
+		for _, value := range values {
+			dst.Add(key, value)
+		}
+	}
+}